A regulatable transformer with semiconductor switches is already described in DE 22 48 166. The secondary winding there comprises a specific number of regulating winding parts, a certain number of which are assembled in a series-connected group of windings, wherein each winding group has two or three regulating winding parts that are connected in parallel. In that arrangement, each winding part is provided with a contactless switch. This reference also describes another variant wherein the secondary winding of the transformer comprises a group of series-connected regulating winding parts, with each winding part comprising four contactless switches. The arrangement is equipped such that terminal voltage polarity of the regulating winding part is reversible and the entire regulating winding part is optionally bypassable as well.
A further apparatus for stepped switching of the secondary voltage of a transformer is known from DE 25 08 013 [U.S. Pat. No. 3,978,395]. The secondary winding is grouped in partial windings in that case too, and semiconductor switches can also be provided.
DE 197 47 712 describes a similar type of tap changer arranged on a tapped transformer that is constructed as an autotransformer. Individual winding parts that are connectable individually and independently from each other, are also provided here. Besides fixed taps of the regulating winding, this arrangement also comprises separate switchable or connectable winding parts.
Known from WO 1995/027931 [U.S. Pat. No. 5,604,423] are various embodiments of another tap changer for uninterrupted load transfer, wherein thyristors also serve as switches. Different winding parts of a tap winding can then be connected or disconnected as part of the secondary winding of the respective tapped transformer by anti-parallel connected thyristor pairs. The specification further proposes a method termed “discrete circle modulation” for realizing as fine-stepped a voltage regulation as possible with a limited number of winding taps present, in which method the thyristors are activated in such a manner that intermediate values of the secondary voltage result.
A modularly constructed semiconductor tap changer system is known from the publication “A new approach to solid-state on load tap changing transformers” (Osman, Demirci; David, A., Torrey; Rober, C., Degeneff; IEEE Transactions on Power Delivery, Vol. 13, Issue: 3, July 1998). There, the regulating winding is connected in series with multiple modules. Each of the modules comprises semiconductor switches and partial windings with different numbers of turns. Targeted actuation of the semiconductor switches allows the partial windings to be connected with or disconnected from the regulating winding. By the different numbers of turns it is even possible to switch partial windings against each other and against the main winding.
In the solutions known from the prior art, semiconductor switches de facto take over the function of the mechanical selector arm in classic mechanical tap changers. Individual winding taps of the regulating windings can specifically be connected or disconnected by the semiconductor switches. It is also possible to subdivide the regulating winding into partial windings that are separately connectable.
These tap changers must comply with the requirements of the different IEC standards 60214-1 in combination with IEC 60060. Among others, a separate source AC withstand voltage test and a lightning impulse voltage test have to be performed. In the separate source AC withstand voltage test, a single-phase alternating voltage of a frequency of 50 or 60 Hz is applied for 60 seconds. According to the approved medium operating voltage, these values can range from 20 kV to 325 kV. The lightning impulse voltage test must be performed at the standard 1.2/50 μs. The alternating voltage can rise up to a level of 1.8 MV in the process. Since these alternating voltages apply immediately at the tap changer, in particular at the semiconductor switches, and since it is either not possible or only possible with great effort to design the semiconductor switches for such high alternating voltages, damages to the semiconductor switches are likely to occur.